In the conventional case, the radiofrequency subsystem comprises two output ports and each feed of the focal array has a corresponding thin beam radiated by the antenna and an area of coverage on the ground called spot. It is possible to obtain a radiation from the antenna by multiple beams if the individual beams are decoupled from one another, the decoupling being able to be spatial or obtained by the use of orthogonal polarizations or of different frequencies between two adjacent beams. The geometrical laws that make it possible to project the desired terrestrial coverages into the focal plane of the antenna and correctly position the phase centre of each primary feed corresponding to each spot. When the coverage is made up of spots evenly arranged on the ground, the difference between two adjacent spots directly imposes the space separating two adjacent feeds in the focal plane.
The creation of a large number of contiguous fine beams entails producing an antenna comprising a large number of individual radiating elements, placed in the focal plane of a parabolic reflector. In the case of a conventional antenna in SFPB (Single Feed Per Beam) configuration corresponding to one feed per beam, the volume allocated for the placement of a radiofrequency RF subsystem responsible for handling the transmission and reception functions in circular bipolarization mode is bounded by the radiative surface area of a radiating element.
In this configuration where each feed, consisting of a radiating element coupled to a radiofrequency subsystem comprising a transmission port and a reception port, generates a beam, each beam formed is transmitted for example by a dedicated horn constituting the individual radiating element and the radiofrequency subsystem handles, for each beam, the transmission/reception functions in single-polarization mode in a frequency band chosen according to the needs of the users. To obtain a good spot radiation efficiency, the horns of the radiating arrays must benefit from sufficient space enabling them to be sufficiently directional in order to light up the edge of the reflectors with sufficiently low levels and thus make it possible to limit the losses by spill-over. Since the spots are interleaved, the space between two feeds of an antenna may not be compatible with the physical dimensions of the horns to achieve the desired radiofrequency performance levels. For example, such is the case for spot sizes less than 1°. To resolve this problem, a choice is generally made to use three or four different antennas which each produce a third, or respectively a quarter, of the coverage. Thus, two adjacent spots of the coverage are not produced by the same antenna. When there is no constraint on the arrangement of the antenna array, this configuration generally makes it possible to obtain very effective antenna performance levels. However, when the diameter of the beams decreases, the geometrical constraints increase and it is not possible to have sufficient space to locate each horn despite the fact that the coverage is shared over three or four antennas. For very fine spots with a size of between 0.2° and 0.4°, the space allocated to each feed of the focal array becomes very small and the reflector is seen by each feed of the focal array at a subtended angle that does not permit the feeds to produce sufficient directivity to avoid the losses by spill-over.
A second antenna configuration that makes it possible to generate a large number of contiguous fine beams is to use a system of two antennas in MFPB (Multiple Feed Per Beam) configuration using a plurality of feeds per beam. Generally, the first antenna Tx operates in transmission mode, the second antenna Rx operates in reception mode, and for each antenna, each beam is formed by combining the signals from a plurality of adjacent individual feeds, some of these feeds being reused to generate contiguous beams. A satisfactory radiation efficiency is obtained by virtue of the reuse of the feeds which participate in the formation of a plurality of beams, which makes it possible to increase the radiative surface area allocated to each beam and to reduce the losses by spill-over. When the feeds are shared between a plurality of beams of the same frequency and of the same polarization, it is possible to create a condition of independence between the beams sharing radiating elements by imposing the formation of so-called orthogonal laws. The orthogonality is produced by using directional couplers which isolate in pairs the distribution circuits of a beam-forming network BFN which share the same radiating elements. However, the constraints of orthogonality cause a deformation of the radiating patterns of the antennas and an increase in the Ohmic losses of the recombination circuits linked to the complexity of the distribution circuits. The aggregate losses are often significant, that is to say of the order of 1 dB. Furthermore, it is necessary to limit the complexity of the beam formers to a rate of reuse of two radiating elements per spot. This leads to physically separating the circuits for combining two adjacent beams by a distance corresponding to two adjacent radiating elements. For spots that have an angular difference of between 0.2° and 0.3°, the apparent focal length may be very great, for example of the order of 10 metres. Finally, the reuse of the feeds during the creation of two adjacent beams presents major drawbacks linked to the bulk of the combination circuits, to the weight of the beam former and to the complexity of the formation of the amplitude and phase laws of each antenna. In practice, for a reuse of two feeds per polarization, the number of individual radiofrequency RF subsystems increases by a factor greater than four with the number of spots to be formed. Thus, for 100 spots, a number of radiofrequency RF subsystems greater than 400 radiation elements is required which necessitates a surface area in the focal plane of the order of 500 mm*500 mm. The weight and the volume of the beam former then become unmanageable.